Overlaying a security document such as a driving licence or passport with a hologram, which is difficult to copy, can afford additional security value. For example in the British passport document, embossed holography has been used in the form of a translucent thin layer coated on its reverse side with a layer of material such as zinc sulphide, whose refractive index is significantly higher than the carrier. Thus light is reflected from the interface in such a way as to reconstruct the holographic image, whilst permitting the viewer to see simultaneously, printed matter behind the holographic film.
The idea is that an attempt to remove the covering layer will result in destruction of the document but in practice one of the most common forms of passport/ID forgery is where the protective laminate is lifted off, allowing the print underneath to be tampered with or the laminate to be reused on other documents. Background prior art can be found in EP 0 754 987A, WO 2006/077447 A2, U.S. Pat. Nos. 4,971,646A, 5,834,096 A and US 2006/0289114 A1.
There is therefore a need for improved anti-counterfeiting techniques for security documents and the like.